The invention relates to a method for producing a bipolar plate comprising a metal-foil strip having a a flow field, inner forms such as holes and slots in the plate, and at least one reference geometry on the outer form thereof, in which the strip is fed to a tool, which opens at the lower dead-center position of the ram of a press, and closes at the upper dead-center position of the ram of the press, is clamped between an upper tool part, which comprises at least a forming punch, a cutting- and piercing punch and a guide and is mounted on the press table, and at least one lower tool part, which performs a stroke between the upper and lower dead-center positions upon closing, and includes a cutting die, the strip finally being subjected to a surface structure for cutting and forming operation.
The invention further relates to an apparatus (in the specification hereafter referred to as a “device”), for producing a bipolar plate comprising a metal-foil strip having a surface structure for a flow field, inner forms such as holes and slots in the plate, and a reference geometry on the outer form thereof, the device comprising a progressive die driven by a ram of a press, the upper tool part thereof, which is mounted on the press table, including at least one piercing and cutting punch and a guide holder, and the lower tool part thereof, which performs a stroke between the upper and lower dead-center positions of the piston, including at least one cutting die.
As is known, bipolar plates for fuel cells comprise an anode side and a cathode side, which are optionally separated from one another by an intermediate wall (interconnection plate). The anode and cathode sides are formed as structured flat plates, the function of which in a fuel cell is to provide an electric connection for the fuel cell and to supply this with hydrogen and air/oxygen as the oxidizing agent. In order to ensure that the fuel cell supply is uniform across the cross-sectional area, the bipolar plate has a flow field formed of a plurality of adjacently disposed channels or grooves, through which the process gases flow along the grooves to the reaction zones, and through which the resultant product water drains off. The bipolar plate comprises holes, which are separated from one another, via which the hydrogen that serves as the fuel, air/oxygen that serves as the oxidizing agent and water that serves as the coolant are simultaneously fed into the respective flow channels. Complex technical requirements are placed on the bipolar plate, namely: that it conduct the gases and current with minimal electrical resistance; have high resistance to aggressive chemical conditions; dissipate the resultant reaction heat; be corrosion-resistant to fuel, oxygen and water; separate the gases from one another; and have high temperature resistance, strong mechanical strength, and dimensional stability, at a low weight and with a small volume.
It is known to use metal foils for such bipolar plates, which are brought into the desired shape via stamping, deep drawing, punching or forming operations based on active means, such as hydroforming (DE 10 2005 021 487 A1, DE 10 2009 059 769, DE 10 2004 016 318 A1, DE 10 2009 036 039 A1). All these known solutions have the disadvantage that the required geometrical trueness cannot be ensured. As a result, an offset arises in the channel geometries and the holes in the bipolar plates, which can lead to safety-relevant malfunctions of the fuel cell. Thermal processes (see DE 10 2010022094A1) regularly necessitate cost-intensive remachining due to the resultant burr.
Furthermore, DE 10 2010 020 178 A1 makes known a method for producing a metallic bipolar plate for a fuel cell stack, which has a gas distribution structure on each of the two sides thereof. The gas distribution structures are created simultaneously on both sides by way of shearing.
This known method is limited to a thickness range of 0.5 to 5 mm, and thus the bipolar plates attain considerable weight and very large dimensions.